immutableadj. not subject to change, either because no operator is provided which is capable of effecting such change or because some constraint exists which prohibits the use of an operator that might otherwise be capable of effecting such a change. Except as explicitly indicated otherwise, implementations are not required to detect attempts to modify immutableobjects or cells; the consequences of attempting to make such modification are undefined. ``Numbers are immutable.''

implementationn. a system, mechanism, or body of code that implements the semantics of Common Lisp.